Instabilities and waves in thin films of living fluids

TL;DR

This paper develops a hydrodynamic model for thin films of living fluids with polar self-driven particles, revealing instabilities influenced by activity, polarity, and free surfaces, relevant to biological systems.

Contribution

It extends thin-film hydrodynamics to include self-propelling particles, predicting instabilities in biological suspensions and structures.

Findings

Instabilities are predicted in bacterial suspensions.

Instabilities may influence biofilm morphology.

The theory suggests experimental tests for validation.

Abstract

We formulate the thin-film hydrodynamics of a suspension of polar self-driven particles and show that it is prone to several instabilities through the interplay of activity, polarity and the existence of a free surface. Our approach extends, to self-propelling systems, the work of Ben Amar and Cummings [Phys Fluids 13 (2001) 1160] on thin-film nematics. Based on our estimates the instabilities should be seen in bacterial suspensions and the lamellipodium, and are potentially relevant to the morphology of biofilms. We suggest several experimental tests of our theory.